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 761 
 WTPZ42 KNHC 271457
 TCDEP2
 
 TROPICAL STORM MARTY DISCUSSION NUMBER   4
 NWS NATIONAL HURRICANE CENTER MIAMI FL       EP172015
 1000 AM CDT SUN SEP 27 2015
 
 First-light visible imagery shows that Marty has developed a
 central dense overcast with a large outer convective band in the
 eastern semicircle.  A recent SSM/IS overpass showed a well-defined
 convective band, but did not show an eye or eyewall underneath the
 overcast.  Various subjective and objective satellite intensity
 estimates range from 35 kt to 65 kt, so the initial intensity is
 raised to 50 kt as a compromise.  The cirrus outflow is good to the
 east but non-existent to the west, likely due to the the effects of
 20 kt of westerly vertical shear.
 
 Marty remains in a moist airmass and over sea surface temperatures
 of near 30C.  However, all of the dynamical guidance suggests the
 cyclone should experience increasing shear during the next 72
 hours. The guidance is in agreement that Marty should strengthen
 for another 24 hours or so, followed by significant weakening.
 Based on this, the new intensity forecast is an update of the
 previous forecast.  However, it should be noted that this is a low
 confidence forecast due to uncertainty over how much land
 interaction may occur as Marty approaches the coast of Mexico.
 
 The initial motion is 015/6.  Marty is located to the west of a
 mid-level ridge which extends across Central America, and to the
 south of a deep-layer trough located over Mexico and the southern
 United States.  The cyclone should turn northeastward at a slower
 forward speed during the next 24-48 hours based on the consensus of
 the track guidance.  After that time, there is some divergence in
 the guidance.  The GFS moves Marty northeastward over Mexico.  On
 the other hand, the ECMWF and UKMET forecast Marty to shear apart,
 with the low-level center turning westward and remaining offshore.
 Based on the guidance and the current trends, the new track
 forecast is similar to the previous forecast in that it keeps Marty
 offshore through the forecast period.  However, it is shifted
 closer to the coast than the previous forecast.
 
 A tropical storm warning may be required for portions of the coast
 of Mexico later today.
 
 FORECAST POSITIONS AND MAX WINDS
 
 INIT  27/1500Z 14.9N 102.8W   50 KT  60 MPH
  12H  28/0000Z 15.4N 102.7W   60 KT  70 MPH
  24H  28/1200Z 16.0N 102.2W   65 KT  75 MPH
  36H  29/0000Z 16.3N 101.8W   65 KT  75 MPH
  48H  29/1200Z 16.7N 101.5W   55 KT  65 MPH
  72H  30/1200Z 17.0N 102.0W   40 KT  45 MPH
  96H  01/1200Z 17.5N 103.0W   30 KT  35 MPH
 120H  02/1200Z 17.0N 104.5W   20 KT  25 MPH...POST-TROP/REMNT LOW
 
 $$
 Forecaster Beven
 
 
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